Escape and evolution of Mars's CO₂ atmosphere: Influence of suprathermal atoms

Abstract

With a Monte Carlo model we investigate the escape of hot oxygen and carbon from the Martian atmosphere for four points in time in its history corresponding to 1, 3, 10, and 20 times the present solar EUV flux. We study and discuss different sources of hot oxygen and carbon atoms in the thermosphere and their changing importance with the EUV flux. The increase of the production rates due to higher densities resulting from the higher EUV flux competes against the expansion of the thermosphere and corresponding increase in collisions. We find that the escape due to photodissociation increases with increasing EUV level. However, for the escape via some other reactions, e.g., dissociative recombination of O2+, this is only true until the EUV level reaches 10 times the present EUV flux and then the rates start to decrease. Furthermore, our results show that Mars could not have had a dense atmosphere at the end of the Noachian epoch, since such an atmosphere would not have been able to escape until today. In the pre‐Noachian era, most of the magma ocean and volcanic activity‐related outgassed CO2 atmosphere could have been lost thermally until the Noachian epoch, when nonthermal loss processes such as suprathermal atom escape became dominant. Thus, early Mars could have been hot and wet during the pre‐Noachian era with surface CO2 pressures larger than 1 bar during the first 300 Myr after the planet's origin.

Origin and evolution of the atmospheres of early Venus, Earth and Mars

“…Due to its low gravity the evolution of Mars' early atmosphere was more sensitive to the EUV flux of the young active Sun compared with Venus and Earth. Depending on the young Sun's EUV activity and related rotation history, Mars could have lost its magma ocean outgassed steam atmosphere early on (Erkaev et al 2014), or may have kept several bar of CO 2 at about 0.1 Gyr (Tian et al 2009;Amerstorfer et al 2017), if one assumes that the young Sun was a slow rotator. Depending on impacts and greenhouse gases (i.e., CH 4 , H 2 , etc.)…”

“…Depending on the absolute amounts of the catastrophically outgassed volatiles (Elkins-Tanton 2008; Erkaev et al 2014) and impacts (e.g., Segura 2002;Rodriguez et al 2015;Iijima et al 2017;Palumbo and Head III 2018), available greenhouse gases (i.e., CO 2 , CH 4 , H 2 , etc.) (Kasting 1991;Forget et al 2013;Ramirez et al 2014;Wordsworth et al 2013;Kerber et al 2015;Wordsworth 2016b;Fairén 2017;Iijima et al 2017) and the Sun's EUV flux evolution, the surface partial pressures of H 2 O, CO 2 , CO, N 2 , and O, large lakes, or an ocean could have been episodically present on Mars' surface during the first ≈100-300 Myr in the early Noachian (e.g., Dohm et al 2008;Buhler et al 2014;Palumbo et al 2014;Rodriguez et al 2015;Rosenberg and Head III 2015;Amerstorfer et al 2017;Fairén 2017;Luo et al 2017;Palumbo and Head III 2018).…”

“…The initial CO 2 partial surface pressure decreased via EUV-driven thermal escape to low values until about 4.0 -4.2 Gyr ago (Tian et al 2009;Amerstorfer et al 2017). Moreover, Zahnle et al (2008) studied the photochemical stability of the Martian CO 2 atmosphere during the planet's history.…”

“…According to the study of Amerstorfer et al (2017) one can expect that nonthermal atmospheric escape processes prevented the build-up of an atmosphere with the upper surface pressure values inferred from the paleopressure study. These results are in agreement with earlier studies such as those of Fox (1993), Jakosky et al (1994), and Hutchins and Jakosky (1996).…”

Origin and solar activity driven evolution of Mars' atmosphere

“…For present Mars we obtain total escape rates for "hot" O atoms of ∼9×10 25 s −1 and for "hot" C atoms ∼2.7 × 10 25 s −1 during high solar activity conditions and ∼3 × 10 25 s −1 for "hot" O atoms and ∼3 × 10 24 s −1 for "hot" C atoms during low solar wind conditions. Monte Carlo calculations based on the thermosphere profiles modeled by Tian et al (2009) during the earlier periods yield an integrated total CO 2 loss from today to 4 Gyr ago of ∼200-400 mbar (Amerstorfer et al, 2017).…”